Step 8: Open iTunes. iTunes will tell you it has detected a device in recovery mode. IMPORTANT: PRESS OPTION AND CLICK RESTORE. Do not simply press restore, this will upgrade your device to stock 4.2.1, and WILL upgrade your baseband (which is irreversible). A dialog box will open and allow you to choose the firmware bundle to restore with. Choose the custom created bundle.

Step 9: Wait for the restore to complete, and enjoy your untethered jailbreak! Unlockers on an unlockable baseband can simply install ultrasn0w and unlock. Enjoy!